1. Field of the Invention
This invention relates to a control circuit for driving an electric motor which is suitable for use, for example, in a magnetic disc storage unit.
2. Prior Art
There has been proposed a disc storage unit in which a seek operation to move the heads to the desired cylinder is performed by means of an electric motor, such as a DC electric motor and a stepping motor. It is known in the art that the electric motor can be controlled by a control circuit in such a manner that the rotor thereof is accelerated and decelerated in accordance with a predetermined unique speed pattern previously stored in an associated memory. However, such a conventional motor control circuit suffers from a deficiency that the rotor does not accurately follow the predetermined speed pattern when the supply voltage for the electric motor becomes less than the nominal value by a certain amount. The reason for this is that the motor is driven in accordance with the same speed pattern regardless of the variation of the supply voltage. For example, in the case of a DC electric motor, the rotor is not properly decelerated because of the lack of the braking force when the supply voltage is excessively low. Similarly, in the case of a stepping motor, the rotor steps out when the supply voltage is excessively low. Thus, in both cases, the low supply voltage results in a seek error in the disc storage unit. The above problems can be solved by lowering the maximum speed taken in the speed pattern of the rotor. This however, prolongs the access time of the disc storage unit.